The present invention relates to processes for making an intraocular implant and the implants thereby made. In particular, the present invention relates to low temperature processes for making implants suitable for intraocular use.
It is known to make drug delivery systems suitable for intraocular use (“implants”). An implant can comprise one or more therapeutic agents as well as one or more biodegradable or non-biodegradable carriers (such as a polymeric or non-polymeric carrier). Typically, the carrier comprises the bulk (i.e. more than 50%) of the implant by weight and can function to hold (the carrier function) and then release the therapeutic agent in vivo, for example as a biodegradable or bioerodible carrier is degraded in situ at or in proximity to the ocular tissue target site. Biocompatible implants for placement in the eye have been disclosed in a number of patents, such as U.S. Pat. Nos. 4,521,210; 4,853,224; 4,997,652; 5,164,188; 5,443,505; 5,501,856; 5,766,242; 5,824,072; 5,869,079; 6,074,661; 6,331,313; 6,369,116; and 6,699,493.
Implants suitable for intraocular use have been made by various methods including compression, solvent evaporation and extrusion methods. An extrusion method for making an intraocular implant can be carried out by first mixing a therapeutic agent with a polymer or polymers. Typically, solid forms (i.e. powders) of the therapeutic agent and the polymers are mixed together to achieve a homogenous mixture of the powders. As noted, the polymer can function as a carrier for the therapeutic agent. Thus, if a biodegradable polymer is used the therapeutic agent can diffuse out of the polymer upon intraocular insertion or implantation of the implant, as the polymer degrades. Although the therapeutic agent-polymer mixture can be compressed to form a tablet, an extruded implant can exhibit a more desirable release profile for the therapeutic agent. Hence, an implant with superior characteristics can be made by heating the therapeutic agent-polymer mixture to the temperature at which the polymer melts, followed by extrusion of an implant with desired dimensions. Melting the polymer helps ensure an even distribution of the active agent within the polymeric matrix and upon cooling provides a solid form implant. It is known to make extruded implants for intraocular use in which the therapeutic agent-polymer mixture is heated to about 90° C. to about 115° C. prior to being extruded. See eg published U.S. patent application number 20050 048099.
Unfortunately heating the therapeutic agent-polymer mixture to a temperature at which the polymer melts can have undesirable or destabilization effects. For example, heating the polymer to its melt temperature can result in the formation of degradation products and/or aggregates of either or both the therapeutic agent and the polymer. This can result in the materials potentially toxic or immunogenic to sensitive ocular tissues and/or can interfere with obtaining a desired release profile of the therapeutic agent from the extruded implant. Additionally, heating the therapeutic agent to the melt temperature of the polymeric carrier (so as to provide a homogenous dispersion of the therapeutic agent in the polymeric matrix) can reduce the potency of a heat sensitive therapeutic agent, thereby reducing the therapeutic efficacy of the resulting implant.
Another problem with existing implants can arise from the presence of polymorphs of the therapeutic agent. A polymorph is a substance which has a chemical composition identical to that of another substance but which exists in a different crystal structure (eg diamond and graphite). Different polymorphs of a substance can have different stabilities, solubilities and, for a therapeutic agent, different potencies or therapeutic efficacies. With known implants, a crystalline therapeutic agent is typically melted along with its polymeric matrix and may recrystallize upon formation of the solid implant. Alternately, the crystalline therapeutic agent can be mixed with the polymer without melting the therapeutic agent. In either case, the therapeutic agent is present in the final implant as crystals (i.e. as particles) of the therapeutic agent dispersed throughout the polymeric matrix. Hence, with either known method for making an implant the therapeutic agent is present in polymorphic forms, each of which therapeutic agent polymorph can have a different therapeutic efficacy.
Hypotensive therapeutic agents are useful in the treatment of a number of various ocular hypertensive conditions, such as post-surgical and post-laser trabeculectomy ocular hypertensive episodes, glaucoma, and as presurgical adjuncts. Glaucoma is a disease of the eye characterized by increased intraocular pressure. On the basis of its etiology, glaucoma has been classified as primary or secondary. For example, primary glaucoma in adults (congenital glaucoma) may be either open-angle or acute or chronic angle-closure. Secondary glaucoma results from pre-existing ocular diseases such as uveitis, intraocular tumor or an enlarged cataract.
The increased intraocular pressure characteristic of glaucoma can be due to the obstruction of aqueous humor outflow. In chronic open-angle glaucoma, the anterior chamber and its anatomic structures appear normal, but drainage of the aqueous humor is impeded. In acute or chronic angle-closure glaucoma, the anterior chamber is shallow, the filtration angle is narrowed, and the iris may obstruct the trabecular meshwork at the entrance of the canal of Schlemm. Dilation of the pupil may push the root of the iris forward against the angle, and may produce pupillary block and thus precipitate an acute attack. Eyes with narrow anterior chamber angles are predisposed to acute angle-closure glaucoma attacks of various degrees of severity.
Secondary glaucoma is caused by any interference with the flow of aqueous humor from the posterior chamber into the anterior chamber and subsequently, into the canal of Schlemm. Inflammatory disease of the anterior segment may prevent aqueous escape by causing complete posterior synechia in iris bombe and may plug the drainage channel with exudates. Other common causes are intraocular tumors, enlarged cataracts, central retinal vein occlusion, trauma to the eye, operative procedures and intraocular hemorrhage.
Considering all types together, glaucoma occurs in about 2% of all persons over the age of 40 and may be asymptotic for years before progressing to rapid loss of vision. In cases where surgery is not indicated, topical beta-adrenoreceptor antagonists have traditionally been the drugs of choice for treating glaucoma.
Some prostaglandins are utility as ocular hypotensive agents, including PGF2α, PGF1α, PGE2, and certain lipid-soluble esters, such as C1 to C5 alkyl esters, e.g. 1-isopropyl ester, of such compounds. Unfortunately, ocular surface (conjunctival) hyperemia and foreign-body sensation have been consistently associated with topical ocular use of prostaglandins as anti-hypertensive agent (i.e. to treat glaucoma), including PGF2α and its prodrugs, e.g. its 1-isopropyl ester. The PGF2α derivative latanoprost is sold under the trademark Xalatan® for treating ocular hypertension and glaucoma. Topical use of latanoprost can have the undesirable side effect of turning the iris of a user brown.
In Laedwif M. S. et al., PROSTAGLANDINS LEUKOT. ESSENT. FATTY ACIDS 72:251-6 (April 2005), it was disclosed that infusion of with a cyclic lipid (prostaglandin E1) in patients with age-related macular degeneration (ARMD) resulted in an improvement in visual acuity.
Bimatoprost is an analog (that is a structural derivative) of a naturally occurring prostamide. The formula for bimatoprost (C25H37NO4) is ((Z)-7-[1R,2R,3R,5S)-3,5-Dihydoxy-2-[1E,3S)-3-hydroxy-5-phenyl-1-pentenyl]cyclopentyl]-5-N-ethylheptenamide. Its' molecular weight is 415.58. Bimatoprost is a heat sensitive molecule, meaning that it can degrade if heated to a temperature greater than about 65° C. In a low pH environment bimatoprost can degrade at a lower temperature and at a faster rate. Bimatoprost has several polymorphic crystal structures. Not all the polymorphs of bimatoprost have the same level of biological activity. Bimatoprost is slightly soluble in water (by definition 3 mg of a water soluble substance can be dissolved in one mL of water at 25° C.).
Bimatoprost can be used to reduce intraocular pressure. See eg Cantor, L., Bimatoprost: a member of a new class of agents, the prostamides for glaucoma management, Exp Opin Invest Drugs (2001); 10(4): 721-731, and; Woodward D., et al., The Pharmacology of Bimatoprost (Lumigan™), Surv Ophthalmol 2001 May; 45 (Suppl 4): S337-S345. An ophthalmic solution of 0.03% bimatoprost is sold by Allergan (Irvine, Calif.) under the trademark Lumigan®. Lumigan® is an effective treatment for ocular hypotension and glaucoma and is administered topically to the effected eye topically once a day. Each mL of Lumigan® contains 0.3 mg of bimatoprost as the active agent, 0.05 mg of benzalkonium chloride (BAK) as a preservative, and sodium chloride, sodium phosphate, dibasic; citric acid; and purified water as inactive agents.
It is known to make bimatoprost containing implants for intraocular use. See eg U.S. patents application Ser. Nos. 10/837,260 and 11/368,845.
Polymer Solubility Parameters
A solubility parameter for a substance is a numerical value which indicates the relative solvency behavior of that substance. The solubility parameter is derived from the cohesive energy density of the substance, which in turn is derived from the heat of vaporization. The heat of vaporization of a substance is the energy required to vaporize (render into a gas) the substance. From the heat of vaporization (in calories per cubic centimeter of a liquid substance), one can derive the cohesive energy density (c):
                    c        =                                            Δ              ⁢                                                          ⁢              H                        -            RT                                V            m                                              (        1        )            where: c=cohesive energy density; ΔHv=heat of vaporization; R=a gas constant; T=Temperature, and Vm=molar volume. The cohesive energy density (c) of a liquid is a numerical value that indicates the energy of vaporization in calories per cubic centimeter, and is a direct reflection of the degree of van der Waals forces holding the molecules of the liquid together. Since the solubility of two materials is only possible when their intermolecular attractive forces are similar, materials with similar cohesive energy density values are miscible in each other.
The square root of the cohesive energy density (c) provides a solubility parameter for a substance:
                    ∂                  =                                    c                        =                                          [                                                                            Δ                      ⁢                                                                                          ⁢                      H                                        -                    RT                                                        V                    m                                                  ]                                            1                /                2                                                                        (        2        )            
This solubility parameter can be represented as delta (δ). δ can be expressed in calories/cc (the standard or older parameter) or in standard international units (SI units). The SI unit is in pascals. Thus, one MPa is one milliPascal. SI parameters are about twice the value of the standard solubility parameter units:δ/cal1/2 cm−3/2=0.48888×δ/MPa1/2  (3)δ/MPa1/2=2.0455×δ/cal1/2cm−3/2  (4)
The newer SI units for the solubility parameter of a substance are usually designated as δ/MPa1/2 (sometimes shown in a shorthand version as just MPa1/2) or δ(SI).
Since a polymer will typically decompose before its heat of vaporization could be measured, swelling behavior is one of the ways that a solubility parameter can be determined for a polymer. The term cohesion parameter can be used to mean the solubility parameter of a non-liquid material. The solubility parameters for biodegradable polymers can be determined. See e.g. Siemann U., Densitometric determination of the solubility parameters of biodegradable polyesters, Proceed Intern Symp Control Rel Bioact Mater 12 (1985):53-54. As noted above, MPa1/2 is a standard unit for solubility parameter. The solubility parameter δ is equal to c1/2, where c=(ΔE/Vm)1/2. In short two materials will mix if their ΔG<0, and ΔG=ΔH−T ΔS (this is the formula for Gibbs Free Energy [ΔG] which defines the free energy of a reaction, where ΔH is the change in enthalpy in a constant pressure process and ΔS is the change in entropy). ΔS is always positive for mixing, but ΔH depends roughly on ΔH˜Vmφ1φ2(δ1−δ2)1/2 where “1” and “2” are the two components. The closer the δ's are to each other, the closer ΔH is to zero and the more energetically favorable the combination.
A solid solution is a solid state solution of one or more solutes in a solvent. A solute initially in a crystalline form which enters into solid solution is no longer in a crystalline form, as is it in a solution, albeit in this case in a solid state solution. Some mixtures will readily form solid solutions over a range of concentrations, while other mixtures will not form solid solutions at all. The propensity for any two substances to form a solid solution is a complicated matter involving the chemical, crystallographic, and quantum properties of the substances in question. For example, solid solutions can form if the solute and solvent have similar atomic radii (15% or less difference), same crystal structure, similar electronegativities and/or similar valance. It is known to compare the solubility parameters of a water soluble drug and a single polymeric excipient to determine if they are miscible in each other so that a glass solution will be formed upon melt extrusion. Forster, A., et al., Selection of excipients for melt extrusion with two poorly water-soluble drugs by solubility parameter calculation and thermal analysis, Int J Pharmaceutics 226 (2001) 147-161. The ability of one solid to function as a cosolvent (i.e. to solubilize) of another solid (i.e. a polymer) upon formation of a solid solution of the two solids can depend upon the ability of the cosolvent to function as a plasticizer of the polymer and/or due to the relative similarities of their solubility parameters.
Polyethylene Glycol
Polyethylene glycol (“PEG”) has the general formula C2nH4n+2On+1, which can be represented as:

Being a polymer, a polyethylene glycol has a glass transition temperature (Tg) (which can be the same as or different from the softening point or the melt temperature of the polymer), as opposed to a true melting point. Within in a certain range the glass transition temperature of a polyethylene glycol increases as its molecular weight increases. For example PEG 400 has a Tg of 4-8° C., PEG 600 has a Tg of 20-25° C., PEG 1500 has a Tg of 44-48° C., PEG 4000 has a Tg of 54-58° C., and PEG 6000 has a Tg of 56-63° C. Poly(ethylene glycol) is non-toxic, water soluble polymer used in a variety of products. For example it is used in laxatives, skin creams and toothpastes.
PEG-3350 [HO(C2H4O)n] is a synthetic polyglycol having an average molecular weight of 3350.
What is needed therefore is a process for making an intraocular implant from a therapeutic agent and a polymer which does not result in or which reduces the occurrence of undesirable therapeutic agent and/or polymer end products or crystalline forms of the therapeutic agent in the implant.